Natural killer (NK) cells are important mediators of defense against infections and cancer. This project has made significant discoveries about innate cytokine pathways regulating individual NK cell responses, i.e. cytotoxicity, proliferation, and IFN-gamma production, and has advanced understanding of endogenous immune responses to viral infections. Unique types of cytokine regulation have been defined in the spleen as compared to the liver, and there are indications that these are in place to preferentially access NK cell-mediated immunoregulatory as compared to direct antiviral effects at the different sites. To thoroughly understand how NK cell are controlled and act to protect the host, as well as evaluate the relative contributions of the different mechanisms to NK cell activation, the studies proposed in this application will expand our work on cytokine regulation of NK cell responses and extend to examine receptor-mediated regulation. The systems to be examined will continue to be infections of mice with murine cytomegalovirus (MCMV) or lymphocytic choriomeningitis virus (LCMV). The project is based on the hypotheses that; (i) innate cytokines and their effects on NK cells are compartmentally regulated; (ii) NK cell IFN-gamma production is tightly controlled in the spleen whereas cytotoxicity is promoted to protect against cytokine mediated disease; (iii) because of importance to defense in the liver, the conditions at this site preferentially promote NK cell IFN-gamma production; (iv) functions of NK cell signaling through positive receptors are to activate NK cells under low levels of innate cytokine induction, and to overcome negative regulation mediated by cytokines; and (v) conversely, positive effects delivered by cytokines overcome negative receptor signaling. These will be tested in four specific aims. Aim 1 will further characterize the cell sources of innate cytokines, their effects on NK cells, and mechanisms for particular compartmental differences. Aim2 will examine the effects of different NK cell responses in protection at the particular sites. Aim 3 will evaluate NK cell responses to, and regulation of, other innate cells during infections. Aim 4 will develop and use unique reagents to dissect the relative contributions of NK cell regulation by cytokines as compared to cell surface receptors for viral products. The work promises to continue to advance basic immunological knowledge and provide insights for therapy.